Acidulating unit



M. D. BROADFIELD AGIDULATING UNIT Original Fi led Jan. 27. 1931 Aug. 6, 1935.

INVENTOR mm H fimqwzaz ATTORNEY Patented Aug. 6, 1935 UNITED STATES 55m u uuum PATENT OFFICE ACIDULATING UNIT Mack D. Broadfleld, Valdosta, Ga.

5 Claims.

This invention relates to the acidulation of phosphate rock and more particularly to an improvement in the apparatus for producing soluble phosphates by the reaction of sulphuric acid upon pulverized phosphate rock.

It has been the usual practice in this art to rapidly mix the sulphuric acid with the ground or pulverized phosphate rock in relatively large batches in a batch mixer, although in some cases it has been suggested that the mixing be carried on continuously. In order to avoid solidification of the materials in the mixer, whether a batch or continuous mixing operation is used, it has been customary to use enough relatively dilute acid in relation to the pulverized rock to form a liquid mass, and to mix the two rapidly and discharge them immediately thereafter, and while still in a liquid condition, into a stationary or moving curing den.

Sometimes a moderate fan draft has been applied during the mixing operation to withdraw vapors liberated in this operation, but in following such practice it has been necessary to use a very low fan draft in order to minimize the loss of rock dust from above the mixture during the rapid mixing.

Because of the fact that the ingredients are mixed very rapidly in the above procedure and are immediately dumped into the den, the main portion of the reaction takes place while the mass is at rest in the curing den. This results in confining a large part of the gases evolved by the reactions, within the solidified mass which is discharged as a porous spongy mass. A portion of this confined gas is liberated when the mass is disintegrated, but a considerable part of it is retained in the material placed on the storage pile for final curing. The gases confined in this manner in the storage pile include fluorides and other ingredients, which appear to form a binder that causes the product to solidify in the storage pile, so that it cannot be readily handled with a phosphate drag unless the product before being placed on the storage pile has been treated for conditioning by any well known means, which do not herein have to be specified.

As indicated above, it has been the practice to use a dilute acid (52 to 53 B.) in order to make the mixture sufficiently liquid to pass readily into the curing den and to prevent it from hardening in the mixer. If stronger acid were used with this procedure, it would be necessary to add the same quantity of liquid, and consequently a greater quantity of actual acid, in order to make the mass sufliciently fluid to be discharged from the mixer. The excess of acid left in the mass in following this operation would be objectionable in the finished product and would make the procedure uneconomical because of waste of acid. Attempts have been made in some cases to correct for this excess acid by adding dry ground phosphate rock to the mass during the curing or after the mass from the den has been disintegrated but this requires an additional step and does not result in complete reaction between the acid and the extra rock dust so added. The use of stronger acid has also been considered objectionable because of the formation of an insoluble coating on the particles, which coating is very diflicultly soluble in the acid and prevents complete reaction with the particles.

It is an object of this invention to provide an improved mixing and kneading apparatus whereby the above objectionable feature may be eliminated. It is a further object to provide means whereby the relative periods of kneading and mixing can be regulated and the working of the mixture in the kneader can be carried to the desired degree. Another object of this invention is to provide improved means for removing the gases liberated when the acid and ground phosphate rock are mixed and kneaded. Other objects will become apparent.

In referring to the invention, reference will be made to the drawing in which Figure 1 is a longitudinal section through a kneader suitable for use in carrying out my improved procedure. Figure 2 is a section taken on the line 22 of Figure 1. Figure 3 is a section taken on the line 3-3 of Figure 1 and Figure 4 is an end view of the mixer from the right hand side of Figure 1.

The apparatus is made up of a lower casing Ill which may be of cast iron and which has a cylindrically shaped bottom with an enlarged section II at one end and a discharge outlet l2 at the other end. A cover l3, which may be of wood, closes the top of the section It) and is provided with inlets H and I5, over the enlarged section II, for introducing acid and pulverized rock, respectively. At the other end of the cover 13 an outlet I6 is provided for the gases liberated during the reaction. This outlet communicates through a pipe I! with a suitable suction fan (not shown). An opening I8 is also provided in the cover l3 at a point near the outlet end of the enlarged section ll.

Two separately operable groups of paddles l9 and 20 are positioned to rotate within the mixing and kneading sections, respectively, of the casing Ill, The paddles 2 0 are fixed to the shaft 2|,

which is supported at one end in the bearing 22 and which is driven by the gear 23 fixed to its other end, which gear meshes with the gear 24 fixed t0 the main drive shaft 25. The drive shaft 25 may be driven at a constant speed by any suitable means (not shown). The paddles l9 are fixed to a sleeve 26 adapted to rotate on the shaft 2|- The sleeve 26 extends through its bearing 21 in the casing l0 and acts itself as a. bearing for the end portion of the shaft 2 I A gear 28 is fixed to the outer end of the sleeve 25, which gear meshes with a gear 29, also fixed to the drive shaft 25.

The paddles I! are set at an angle to the axis of the sleeve 26 so that a clockwise rotation of these paddles, as viewed in Figure 3, will not only mix the pulverized rock dust and acid, but will advance it toward the kneader section. The paddles 20 are also positioned at an angle so that a clockwise rotation of the shaft 2| will advance the mass while the reacting mixture is agitated and mixed. Some of the paddles, for example, paddles 20a, may be set at a reverse angle so that they will retard the progress of the mass through the kneader. The particular arrangement of the paddles H3, 20 and 20a and the gearing for driving the shaft 2| and sleeve 26 may be adjusted to give the desired rate of mixing and discharge and to mix and knead the mass for the desired length of time. Also the shape of the paddles and the contour of the casing I!) may be modified to give the desired mixing and kneading action. It may be desired to extend the length of the paddles so that they approach the casing more closely and have a greater scraping action. The numbers of paddles reversed in direction may be such that the desired reversing travel of a portion of the mass is obtained to give the proper working and kneading of the mass.

In operating according to my improved process with the apparatus described above, the shaft 2| and the sleeve 26 are rotated in a clockwise direction at the desired speed and preferably with the sleeve operating at a higher rate of speed to assure rapid mixing of the pulverized rock and acid, for example, the shaft 2| may rotate at about 65R. P. M. and the sleeve 26 at about 130 R. P. M. Th'acid and pulverized'phosphate rock are fed continuously through the feed pipes l4 and I5 at the rate of about I50 pounds of 56 to 57 B. acid and about 200 pounds pulverized phosphate rock per minute. These may be supplied to the mixer as described in my copending application 418,455, filed January 4, 1930, of which this application is a continuation, wherein the acid is continuously supplied in measured quantities through a measuring valve in a constant head supply tank and the pulverized rock is supplied substantially continuously through a rotating pocket-type of measuring feeder. Of course, other means may be used for continuously feeding the rock dust and acid.

The rotation of the paddles l9 rapidly mix the rock dust and acid which are retained in the enlarged section of the casing ill a sufficient length of time to thoroughly mix them. The mixture is then advanced into the kneader where the paddles 20 continuously mix and knead the mass as they advance it toward the discharge I2. The paddles 2D and 20a are arranged so that the mass is discharged from the kneader in such a condition that it is substantially ready to solidify, due to the formation of hydrated calcium sulphate from the reaction of the acid upon the tri-calcium phosphates. In the operation described above, I prefer to retain the mass in the enlarged portion of the casing I0 about 2 minutes and in the kneader about 3 minutes.

The kneaded mass is discharged continuously into a curing den which may be a stationary den or a moving den, for example, of the type described in my above mentioned copending application 418,455. If a continuously moving den is used, the solidified mass may be disintegrated at the discharge end of the den by means of a revolving cutter or other suitable means.

During the operation of the mixer the mass is maintained at about the level of the shaft 2|. The gases evolved during the mixing are withdrawn through the pipe I! by a strong fan blast, the opening l8 permitting the entrance of a stream of air which assists in sweeping out the gases. With this arrangement a strong fan blast may be used without loss of rock dust because the blast is applied with greatest force to the material after the rock dust has become mixed with the liquid to form a wet mass, and any dust that may be drawn from the stream of pulverized rock entering the mixer will be removed by the subsequent contact with the agitated mass thrown up by the revolving paddles. The curing den and cutter may be in an enclosure communicating with the discharge opening l2, in which event the fan blast will draw oiT any slight amount of gas liberated in the den and during the cutting operation.

Because of the fact that the gases are liberated during the kneading and are completely withdrawn by a strong fan blast, very little gas is retained in the mass discharged from the curing den and this mass, instead of being a porous mass, is a dense mass which does not require any further step for conditioning and can be readily handled with a phosphate drag.

The working and kneading of the mass during the reaction gives an opportunity for intimate contact between the acid and particles of phosphate rock and causes an abrasive action between the particles of phosphate rock which minimizes the formation of insoluble coatings on the particles, and so permits a more complete reaction to take place.

In following my improved procedure, the ratio of rock dust to acid (expressed as 50 B. acid) is higher than in the usual procedure, for example, I have used a ratio of 1.15 pounds of rock dust to one of acid as compared to 1.04 pounds to one in the old procedure. The thorough mixing and kneading of a greater proportion of pulverized phosphate rock with the more concentrated acid results in the production, from a given phosphate rock. of a product containing a higher percentage of available phosphoric acid (P205). For example, I have used phosphate rock containing 68% bone phosphate of lime (31.12% phosphoric acid calculated as P205). Consequently 1.15# of this rock contains about .358# of P205. For this amount of phosphate rock I have used the equivalent of 1# of 50 B. acid which contains about .203# of combined sulphur, giving a ratio of about 1.76# of P205 to It of sulphur, and as my conversion of insoluble phosphate to soluble or available phosphate was not less than 97%, the ratio of available phosphoric acid (as P205) to sulphur in the resultant product was not less than about 1.7 to 1 as compared to 1.54 to 1 in the prior practice. Also, since less water is present in the ingredients and a more thorough separation of the steam during the reaction is provided, the product is gee. AGlTATlNG- drier than that produced by the usual procedure. The smaller quantity of water present during the reaction also results in a higher temperature in the reacting mass and an increase in the speed of the reaction.

The apparatus and procedure may, of course, be modified and it is not intended to limit it to the particular embodiment described. For example, the initial mixing may be carried out in a batch mixer, after which the mixture can be passed through a kneader where it is agitated and kneaded until the desired reactions have taken place. Also the form of the casing for the kneader and the shape of the blades may be varied, for example the blades may be arranged spirally around the shafts as shown in Figure 1 of application 418,455 and a suitable continuously or intermittently operating scraping device may be added if found necessary.

The terms used in describing and claiming the invention have been used in their descriptive sense and not as terms of limitation and it is intended that all their equivalents be included within the scope of the appended claims.

This application is a division of my copending application Serial No. 511,473, filed January 27, 1931.

What I claim is:

1. An apparatus of the class described, comprising a casing having a mixing chamber and an elongated kneading chamber, a shaft extending through said chambers carrying paddles positioned to advance and knead materials in said kneader and having paddles positioned to retard the forward motion of said materials, means for introducing materials to the mixing chamber. a mixer in said mixing chamber adapted to rapidly mix the materials introduced, and means for drawing a current of gas over the surface of the body of material being kneaded in the kneader and discharging it from the casing.

2. An apparatus of the type described, comprising a substantially horizontal mixing chamcaicu not...

ber having paddles adapted to rotate therein and to mix and advance the material, a kneading chamber communicating with the mixing chamber but separated from itby a baifle, paddles in said kneading chamber adapted to advance the mixture toward an outlet and other paddles adapted to retard the forward movement of the mixture, an opening to the air near the discharge end of the mixing chamber and an air withdrawal means connected to the kneading chamber above and near the outlet.

3. An apparatus of the type described, comprising a substantially enclosed mixing chamber having paddles adapted to rotate therein and to mix and advance the material, a substantially enclosed kneading chamber communicating with the mixing chamber, said kneading chamber having paddles therein adapted to advance the mixture toward an outlet and other paddles adapted to retard the forward movement of the mixture, an opening to the air near the discharge end of the mixing chamber and an air withdrawal means connected to the kneading chamber above and near the outlet, such openings being so arranged that a stream of air may be drawn over the surface of the material in the kneading chamber.

4. An apparatus for producing soluble phosphate from phosphate rock and acid, comprising a mixing chamber communicating with a kneading chamber and separately operated stirring devices having paddles adapted to mix and knead the ingredients in said respective chambers and to progressively feed the mass to a point of discharge, the stirring devices in said kneading chamber being provided with paddles adapted to reverse the flow of a portion of the mixture as it progresses through the kneading chamber.

5. An apparatus as defined in claim 4 in which the separately operated stirring devices are carried on concentric shafts adapted to be rotated at different speeds.

MACK D. BROADFIELD. 

